The objective of this research is to learn how the insertion of a transposable element into a locus can affect gene expression. An understanding of this phenomenon is relevant to human health because some retroviral insertions are known to activate genes that lead to the induction of tumors. Transposable element insertions might also account for the altered patterns of gene expression in development that is associated with some human disorders. We propose to examine a situation in which a gene is turned either "off" or "on" depending on the internal rearrangements within the transposable element. The work is being carried out in maize because of the excellent genetics available in this system and because the organism readily permits the selection of rare phenotypic events. The original mutation studied was selected by McClintock and is a Ds insertion into the Shrunken gene which encodes the enzyme sucrose synthetase. Recombinant DNA techniques and Southern blotting analyses showed that the original mutant allele contains an insertional element of about 20 kb inserted within an intervening sequence. During subsequent reversion and remutation the element remains in the same site and in the same orientation but apparently has undergone complex internal rearrangements. These rearrangements have consequences for transcription of the gene: in the original mutation no detectable transcript is made; in the secondary mutation only the portion of the gene 5' to the site of insertion is transcribed; and in a revertant the full length message is made. The changes seen here resemble the "changes of state" seen with other transposable element-controlled alleles. The altered elements will be isolated and analyzed in detail by restriction analyses and sequencing of pertinent regions to determine the nature of the changes they have undergone. Other controlling element-induced mutations and their revertants will also be examined for their structural changes and physiological effects.